Mind the data: pitfalls and perspectives in acquiring and interpreting preclinical evidence for the FLASH effect

FLASH radiotherapy (FLASH-RT) has rapidly emerged as a promising approach to widen the therapeutic window through differential sparing of normal tissue. However, despite substantial experimental activity, uncertainties remain regarding the magnitude, reproducibility, and determinants of the FLASH effect. This work reviews common pitfalls and sources of ambiguity in the acquisition and interpretation of preclinical FLASH data to support clinical application. In particular, the widespread use of fixed single-dose study designs, over-reliance on differential endpoint-based metrics, and limited quantification of effect size may obscure the true magnitude and clinical relevance of observed effects. Moreover, small dose-modifying effects are highly susceptible to experimental and biological variability, including dosimetric uncertainties, endpoint sensitivity, and environmental factors such as oxygenation. Heterogeneity in experimental protocols and endpoints further complicates cross-study comparisons. Addressing these challenges through improved quantitative characterisation, harmonised methodologies, and transparent reporting will be essential to enable robust interpretation and support the clinical translation of FLASH-RT.